This invention provides a process for the removal of iron- and rhodium-containing catalyst residues from hydrogenated nitrile rubber.
Polymer hydrogenation, and the subsequent separation of the hydrogenation catalyst from the polymer, are both well known unit operations, as disclosed, for example, in U.S. Pat. Nos. 4,396,761; 4,510,293 and 4,595,749.
More specifically, certain rhodium-containing catalysts are known to be particularly suitable for the selective hydrogenation of nitrile rubber (i.e. reduction of the carbonxe2x80x94carbon double bonds without concomitant reduction of the carbon-nitrogen triple bonds present in nitrile rubber). Such hydrogenated nitrile rubber is less susceptible to heat-induced degradation in comparison to unsaturated nitrile rubber.
For example, UK Patent 1,558,491 teaches the use of chlororhodium-(tristriphenylphosphine), [i.e. RhCl(PPh3)3] in such a process. Similarly, U.S. Pat. No. 4,464,515 teaches the use of hydridorhodium-tetrakis(triphenylphosphine) catalyst [i.e. HRh(PPh3)4] for the same purpose. In both of these processes the unsaturated nitrile rubber is first dissolved in a suitable solvent to provide a viscous rubber solution. The catalyst is then dissolved in the rubber solution. These hydrogenation processes are said to be homogeneous because the substrate and catalyst are contained in the same phase.
An advantage of the above homogeneous processes is that they require minimal amounts of catalyst to effect the hydrogenation. However, a major disadvantage of such processes is that it is difficult to remove the catalyst from the reaction mixture once the reaction is complete (by comparison, in a heterogeneous process (i.e. where the catalyst is not dissolved in the reaction medium) the catalyst may be readily removed by filtration or centrifugation).
Besides rhodium, iron residue may also be present in the nitrile polymer. Both iron and rhodium are active catalytic metals and, therefore, it is desirable to remove them from the hydrogenated rubber in order to improve the overall quality of the product. Furthermore, the high price of rhodium provides an economic incentive for its recovery. The prior art directed towards the recovery of rhodium from hydrogenated rubber is disclosed in U.S. Pat. No. 4,985,540, which describes a process in which a solution containing hydrogenated nitrile rubber in a hydrocarbon solvent is treated with an ion-exchange resin. The ion-exchange resin utilized was characterized as being a heterodispersed macroporous resin having a functional group selected from a primary amine, a secondary amine, a thiol, a carbodithioate, a thiourea, and a dithiocarbamate.
The recovery of rhodium complexes from non-viscous chemical process streams using ion-exchange resins is also known. For example, Chemical Abstracts (CA) 75: 19878e (1971) describes the separation of rhodium-containing catalysts from oxo reaction streams using an ion-exchange resin. CA 85: 588k (1976) teaches the use of a thiol-functionalized resin to recover Group VII metals from spent organic solutions which contain catalysts. CA 87: 26590p (1977) describes a two-stage process in which (I) an aqueous, noble-metal containing solution is prepared by extracting metal from a catalyst carrier and (ii) the noble metal is adsorbed by an ion-exchange resin. Finally, CA 95: 10502r (1981) relates to the recovery of platinum and rhodium by extracting the metals from spent catalysts using HCl and HNO3, followed by the subsequent use of an ion-exchange column to separate the metals.
Notwithstanding the above methods of the art, there remains room for improvement in methods for removing iron- and rhodium-containing catalyst residues from hydrogenated nitrile rubber, particularly with respect to viscous solutions of hydrogenated nitrile rubber.
An improved process for the removal of iron- and rhodium-containing catalyst residues from hydrogenated nitrile rubber is provided, the process comprising the treatment of a solution of hydrogenated nitrile rubber containing such residues with an ion-exchange resin, the resin being a homodispersed macroporous cross-linked styrene-divinylbenzene copolymer resin having thiourea functional groups.
The aforementioned ion-exchange resin is capable of removing both iron and rhodium residues from the hydrogenated nitrile rubber.
In a further aspect of the invention there is provided a column process for the removal of iron- and rhodium-containing catalyst residues from hydrogenated nitrile rubber which results in a markedly lower pressure drop across the system, thus increasing production capacity by allowing higher volume throughput.